fix: make values containing backslashes round-trip through set_key

[dchaudhari7177]

Fixes #661.

Problem

set_key writes single-quoted values and escapes single quotes, but not backslashes. The single-quoted-value parser decodes \ and \', so a value containing a backslash does not survive a write/read round-trip:

>>> dotenv.set_key(".env", "PATH", r"C:\Users")   # file gets PATH='C:\Users'
>>> dotenv.get_key(".env", "PATH")
'C:Users'
>>> dotenv.set_key(".env", "RE", r"\d+")
>>> dotenv.get_key(".env", "RE")
'd+'

Windows paths and regular expressions are the common victims.

Fix, part 1 — escape on write

value_to_set.replace("\\", "\\\\").replace("'", "\'"). Backslashes first, so the backslash introduced by the quote escaping is not escaped again.

Fix, part 2 — the write side alone is not enough

With only part 1, a value ending in a backslash is still broken, and breaks its neighbours too:

A='back\'
B='sentinel'

_single_quoted_value was '((?:\'|[^'])*)' — it knows \' is an escape but not \. So in back\' the second backslash pairs with the closing quote as \', the match keeps going, and since [^'] also matches newlines it runs into the next line. The binding then fails _end_of_line, error recovery kicks in, and B is lost as well.

Both quoted-value patterns now say "a backslash escapes the next character":

_single_quoted_value = make_regex(r"'((?:\.|[^'\])*)'", extra_flags=re.DOTALL)
_double_quoted_value = make_regex(r'"((?:\.|[^"\])*)"', extra_flags=re.DOTALL)

re.DOTALL keeps multi-line quoted values working, which the old [^'] allowed.

This is not a behaviour change for anything that parsed correctly before — the captured group is identical for every input the old pattern handled, and decode_escapes is untouched. It only changes inputs that previously failed to parse. Hand-written A='C:\Users\name' still yields C:\Users\name, and A='b\'c' still yields b'c.

Tests

• test_set_key gains three cases pinning the written form for b\c, b\ and b\'c.
• New test_set_key_round_trips parametrizes ten values (Windows paths with and without a trailing separator, regexes, quotes, empty) and asserts both that the value comes back intact and that a following key is still readable — that second assertion is what catches the line-swallowing.
• test_parse_stream gains five cases for escaped backslashes in both quote styles, including the two-binding case.

pytest tests/ passes. The two test_cli.py::test_run_* failures on my machine are pre-existing and Windows-specific (they reproduce on an unmodified checkout).

🤖 Generated with Claude Code

[theskumar]

Hey @dchaudhari7177 - merging this now, thank you! This was the most complete take on #661 by a good margin: escaping on write plus fixing the parser so a trailing backslash can't swallow the next line, and covering both single- and double-quoted values. The test that writes a sentinel key right after the value was a nice touch for catching the desync. Really appreciate the clear writeup too - thanks for the fix!